Significance of Daily Quality Assurance Scan in Hardware Artifact Evaluation.

A failure in any of the hardware components can lead to degradation in image quality and accuracy of quantification. Few artifacts may bring serious impacts on the image quality and finally lead to the wrong diagnosis. We encountered a sudden appearance of the cold area, being more prominently visible on Positron Emission Tomography image in comparison to the fused image. A daily quality assurance scan was performed to evaluate the status of the machine after giving a restart to the system. It was discovered that the module number 9-11 showed dark black color in the graph due to scarcity of counts indicating a failure/malfunctioning of the detector or electronic board-Cassette Electronic Module board (CEM board) supporting the module 9-11. Further evaluation of the system helped us diagnose that one of the electronic board (CEM Board) had become nonfunctional. A new functional electronic board (CEM board) was ordered and placed and the error rectified.

Introduction

Positron emission tomography (PET) is a nuclear imaging technique that uses the unique decay characteristics of radionuclides that decay by positron emission, leading to the production of pairs of gamma rays through annihilation. Following the detection of an annihilation event, the generated signal is processed through a series of electronic boards to determine the energy, position, and timing of the event in a PET scanner. A failure in any of these hardware components leads to degradation in image quality and accuracy of quantification.[1] At a remote place like ours, it becomes difficult to reappoint the patients coming from far off distance in case of any major hardware breakdown. Defective detector blocks in PET may cause serious image artifacts.[2] In the meantime, one must decide whether to proceed with routine patient scanning when there is a known defective detector in the system.[3] Some minor artifacts do not have a final impact on the diagnosis and easily recognized by an expert. Few artifacts may bring serious impacts on the image quality and finally lead to the wrong diagnosis.[4] Daily quality assurance (DQA) tests should be performed religiously and inspected very closely in spite of all parameters being within acceptable range on a PET scanner.

Case Report

We encountered an unusual scenario in our nuclear medicine department. In the afternoon after initial three patients were successfully scanned on our PET/computed tomography (CT) scanner (Discovery STE, GE Medical Systems, USA), all of a sudden there was an appearance of cold area, clearly visible on the upper left side of axial images of PET [Figure 1], which was masked on PET-CT fused images [Figure 2]. We thought of a possibility of motion artifact and tried to proceed further as it was not significantly hampering the clinical interpretation, but the artifact reappeared in the following study and showed a significant reduction in counts at the same side. Since the error was limited to PET image, it was clear that something is reducing the counts in the left side of the gantry. We went inside the gantry room to inspect for any material being stuck up at the PET Mylar window which could be the cause of attenuation, but it was fine. We gave a restart to the system as a routine protocol in case of any technical/software error encountered and acquired DQA scan, with the inbuilt Ge-68 rod source to visualize the change if any in the graph. We discovered that the module number 9–11 showed dark black color in the graph [Figure 3] due to scarcity of counts possibly due to failure/malfunctioning of the detector or electronic board (Cassette Electronic Module board [CEM board]) supporting the module 9–11. A call was registered immediately at the Equipment service center, and an engineer summoned on a priority basis. It was discovered that the electronic board (CEM board) supporting the module 9–11 was malfunctioning. To reconfirm the fault being limited only to electronic board (CEM board), we assigned the same electronic board (CEM board) to module 33 and 34. Moreover, the error shifted to different module [Figure 4] in the DQA graph acquired with Ge-68 rod source, helping us confirm with failure only in electronic board (CEM board) rather than detector module. After replacement of the malfunctioning electronic board (CEM board) with the new functional one, the problem got resolved, and all parameters were in acceptable limits as can be seen in the DQA graph [Figure 5].

Figure 1

Artifact as seen in axial images

Figure 2

(a) Computed tomography image. (b) Positron emission tomography image showing loss of counts in image on the left upper region. (c) Positron emission tomography–computed tomography fusion image. (d) Maximum intensity projection image of the study

Figure 3

Daily quality assurance images as seen after a restart of the system, showing a defect in module 9–11

Figure 4

Daily quality assurance image showing the same defect being shifted to module 33 and 34 after assigning the malfunctioning electronic board (cassette electronic module board) to module 33 and 34

Figure 5

Daily quality assurance image after replacement of faulty electronic board (cassette electronic module board) with a new and functional one, showing complete acceptance of all parameters

Discussion

The electronic board (CEM board) is primarily responsible for singles detection and processing. Each CEM board is capable of processing 24 blocks. The Discovery STE (GE Medical Systems, USA) used at our institute contains a total of 280 blocks, which require 12 CEM board. The CEM board accepts the output of the preamplifier function and converts it to a digitized, qualified and time stamped crystal event. The resulting single events are then converted to a serial LVDS (low-voltage differential signaling) bitstream and transmitted to the Control Program for Microcomputers board for coincidence processing.

Conclusion

DQA scan performed with the Ge-68 rod source can help us identify many significant parameters, and it should be inspected closely for better understanding of the PET scanner hardware.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.